This invention relates in general to the field of directional couplers and, in particular, to directional couplers which incorporate the function of impedance transformation.
Branch line hybrid couplers are often used in a typical transistor amplifier combiner or balanced mixer. These couplers are traditionally designed with four, 50-ohm ports to match the system impedance. However, radio frequency (RF), microwave, and millimeter-wave frequencies active devices used in power amplifiers, mixers, or other applications in which the coupler may be found may present impedances in the range of approximately 1 to 200 ohms.
The range of impedances which the coupler must accommodate typically necessitates the use of impedance transformers that, in either the distributed or lumped domain, increase the size, weight and insertion loss of the device.
Thus, what is needed is a practical, economical directional coupler which incorporates the function of impedance transformation. The relatively small size transforming directional coupler would be realizable in monolithic microwave integrated circuit (MMIC) form.